INTERNAL ANATOMY OF INSECTS. 



larva of a saw-fly in which this part is formed of four 

 nearly equal spherical bulbs a : in the Scorpion (to judge 

 by the figure of Treviranus b ) the two lobes represent 

 an equilateral triangle, the exterior angle of which ter- 

 minates in several lesser spherical bulbs ; in Acrida viri- 

 dissima, Nepa cinerea, Clubiona atrox, and the common 

 Louse, the lobes are pear-shaped e . 



ii. TJie spinal marrow and its ganglions d . From the 

 posterior part of the brain of insects, but in the ground 

 and water beetles (Eutrechina and Eunechma) from its 

 sides below e , issue two chords which diverging embrace 

 the oesophagus, and dipping below it and the intestines, 

 a situation they maintain to the end of their course, 

 and in their further progress uniting at intervals and 

 dilating into several knots or ganglions, compose their 

 spinal marrow. This part is so named, from a supposed 

 analogy to the spinal marrow of vertebrate animals, 

 which however admits of some degree of doubt ; yet, 

 since it mixes the functions of that organ with those of 

 the great sympathetic nerves, the denomination is not 

 wholly improper, and may be retained. Though this 

 chord is usually double when it first proceeds from the 

 brain, and surrounds the oesophagus like a collar, yet in 

 some insects it may be called a single chord. This is 

 the case with that of the common louse, in which Swam- 

 merdam could perceive no opening for the transmission 

 of the part just named f ; if he was not mistaken in this, 

 the brain, as well as the rest of the spinal marrow in 



a Cuv. Anat. Comp. 324. b Arachnid, t. If. 13. m.m. 



c Cuv. ubi supr. 343. 346. Treviranus Arachnid, t. v. /. 45. a. 

 PLATE XXI. FIG. 8. a. 



d Ibid. FIG. 1. b.b. e Cuv. ubi supr. 337. 



f PLATE XXI, FIG. 8. Swamm. Bibl. Nat. \. 36. b. 



